1. Field of the Invention
The present invention generally relates to an injection molded resin gear and an injection molded resin rotating body. More specifically, the invention relates to an injection molded resin gear for use in power transmission devices for various automotive parts, industrial equipments and precision instruments, and an injection molded resin rotating body, such as an injection molded resin gear, sprocket, pulley or roller.
2. Description of the Prior Art
In recent years, automotive parts operating by driving force from a driving source, such as a motor, has many resin gears, which are formed by injection molding, in order to reduce the weight and operation noises thereof.
FIG. 17 is a longitudinal section of such an injection molded resin gear 50, and FIG. 18 is a left side view of the injection molded resin gear 50 of FIG. 17, which is viewed from the left side of FIG. 17. The injection molded resin gear 50 shown in FIGS. 17 and 18 is mold by injecting a molten resin material into a cavity, which has the same shape as that of the gear 50, from one or a plurality of (three in FIG. 18) pin point gates 51.
However, if the resin material is injected into the cavity from the three pin point gates 51 as shown in FIG. 18, parts of the resin material injected from the respective pin point gates 51 meet each other in a substantially middle portion between adjacent two of the pinpoint gates 51 as shown by arrows, to produce weld lines 53 on a web 52. If the weld lines 53 are produced on the injection molded article (the injection molded resin gear 50), there are some cases where the weld lines 53 do not only have a bad influence on the precision of tooth profile, but they also have a bad influence the strength of the gear. In particular, if the resin material to be injected is a reinforced plastic containing glass or carbon fibers, it is known that the strength of the gear is deteriorated by 40 to 60%.
Therefore, as shown in FIG. 19, there has been developed an injection molding technique for using a ring-shaped gate 54 to inject a resin material from the ring-shaped gate 54 to allow the resin material to flow in a cavity in radial directions from an inner peripheral end toward an outer periphery to prevent the occurrence of weld lines 53 shown in FIG. 18. This injection molding technique is often used for molding a gear of a fiber reinforced plastic.
However, if the ring-shaped gate 54 is used for injection molding, the amount of the resin material remaining in the ring-shaped gate 54 after injection molding is larger than that if the pin point gates 51 are used for injection molding, and the amount of wasteful resin material (resin material which is not used as a product and which is cut off from the product) is larger than that if the pin point gates 51 are used for injection molding, so that the yield of the resin material is deteriorated. As a result, the price of the product (the injection molded resin gear 50) formed by injection molding using the ring-shaped gate 54 is higher than the price of the product (the injection molded resin gear 50) formed by injection molding using the pinpoint gates 51. In particular, if an expensive resin material containing reinforced fibers is used as the resin material for injection molding, the difference in price between the products formed by injection molding using the ring-shaped gate 54 and pin point gates 51 is remarkably great.
In addition, in the case of the product (the injection molded resin gear 50) formed by using injection molding the ring-shaped gate 54, after the ring-shaped gate 54 is cut off, it is require to machine and scrape the cut scar of the ring-shaped gate 54 off, so that there is a problem in that the working cost is higher than that in the case of the product (the injection molded resin gear 50) formed by injection molding using the pin point gates 51. Furthermore, in the case of the injection molded resin gear 50 formed by injection molding using the pin point gates 51, the opening area of the pin point gates 51 is small, and the pin point gates 51 are easily removed from the product (the injection molded resin gear 50) when the die is released after injection molding. Therefore, there are advantages in that the cut scars of the pin point gates 51 are inconspicuous, so that it is not required to carry out subsequent working.
In order to eliminate such problems, as shown in FIG. 20, there has been developed a technique wherein a gear 61 molded by injecting a resin material containing reinforced fibers from a plurality of pin point gates 60 is formed with a plurality of web portions 62 and radial ribs 63, and the plurality of pin point gates 60 are arranged in every other positions between adjacent two of the radial ribs 63 for dividing and controlling the flow of a molten resin material, which is injected into a cavity from the pin point gates 60, to equalize the orientation of the resin material to improve the precision of shape and dimension (see Japanese Patent Laid-Open No. 2-44701).
Also, as shown in FIGS. 21 and 22, there has been developed a technique wherein a polymeric material flow regulating mechanism 72 for reducing the differences in filling time and pressure during an injection molding process is formed inside of a tooth portion 71 in the vicinity of teeth on both sides of a gear 70 (see Japanese Patent Laid-Open No. 8-132542). According to this technique, a resin material injected from pin point gates 73 reaches the polymeric material flow regulating mechanism 72 to raise its internal pressure to flow into an unfilled portion, in which pressure is low, to be filled in the polymeric material flow regulating mechanism 72 over the whole circumference thereof, and the resin material flows toward the periphery of teeth when the internal pressure is uniform. Furthermore, the polymeric material flow regulating mechanism 72 is a groove formed in the vicinity of the tooth portion 71 inside thereof, and the groove depth thereof continuously or intermittently varies.
However, the techniques disclosed in Japanese Patent Laid-Open Nos. 2-44701 and 8-132542 can not sufficiently reduce the weld lines 53 which are shown in FIG. 18 and which are produced in the joining portions of parts of the resin material, which are injected from adjacent two of the pin point gates 51, on the web 52 for connecting a rim 55, on which teeth 55 are formed, to a shaft supporting portion 57 into which a shaft is fitted, so that it is not possible to satisfy the strength of the web 52 and the precision of tooth profile to such an extent that they are required on the market in recent years.